The present invention is directed to composition and methods for electrodepositing zinc, and more particularly, to the use of certain additives in such baths to reduce the interference of heavy metal impurities with the electroplating function. Also, these additives brighten the electrodeposited zinc, and also serve to refine the grain structure of the zinc deposit. Preferably, the zinc plating bath is of the alkaline type, containing essentially zincate ions and an alkali, however, improved results may also be obtained by the incorporation of the additives in zinc plating solutions which contain cyanide.
It has been proposed in the prior art, as exemplified by U.S. Pat. No. 3,856,637, to add inorganic silicates to a low cyanide zinc plating bath containing less than about 15 grams per liter of free cyanide. Such soluble inorganic silicates generally include sodium silicate, sodium disilicate or sodium metasilicate. The prior art literature attributes the efficacy of the inorganic silicates as additives to some unexplained effect upon the water used either to make up the bath or to rinse the part to be plated. This same prior art indicates that the baths containing inorganic silicates will give essentially the same results as baths which were made up using distilled water, although free of the silicate additives.